1. Field of the Invention
The invention relates to an image reading apparatus for converting an original put on an original supporting plate into an image signal and for zoom processing the image signal.
2. Related Background Art
Hitherto, an image processing apparatus such as a digital color copying machine or the like has widely been known.
In such an image processing apparatus, a light is illuminated to an original by moving an optical system, the reflected light is read by a photoelectric converting device such as a CCD or the like, and various processes are performed on the resultant electric signal, so that an image process such as enlargement, reduction, or the like can be performed.
On the other hand, a 3-line sensor is frequently used as a CCD of the reading system. In the 3-line sensor, the light components separated into R (red), G (green), and B (blue) are photoelectrically converted every line. For instance, when the image of the original is red, the signal of only the R sensor is generated. In case of the black and white monochrome image, the signal of almost the same level is generated from each of the R, G, and B sensors. Each line device is arranged in parallel with the main scanning direction perpendicular to the moving direction of the optical system.
In the above zooming process for enlargement or reduction, the zoom in the main scanning direction is realized by interpolating each pixel (in the enlarging mode) or thinning out each pixel (in the reducing mode) by using the signal read by the CCD.
The zoom in the subscanning direction (line direction) as a moving direction of the optical system is realized by slowly (in the enlarging mode) or quickly (in the reducing mode) scanning the reading optical system for the original and thereby changing a scanning line width on the original corresponding to one line. According to such a method, a control speed can be finely adjusted within a control range of a drive motor of the optical system and a relatively stable image signal can be obtained.
By using a page memory, on the other hand, the zoom in the subscanning direction can also be performed by using a circuit similar to the zooming circuit in the main scanning direction. In this instance, the zoom is realized by storing the image signal while thinning out the pixels (in the reducing mode) or by interpolating the pixels while reading out the image from the memory (in the enlarging mode) for the page memory. By using such a method, the scanning speed of the optical system can be always made constant.
According to the zooming method of controlling the scanning speed of the optical system as mentioned above, however, there is an allowable range in a rotational speed of the drive motor of the optical system. For instance, since there is a limit in the high speed rotation which is necessary in the reducing mode from a viewpoint of the characteristics of the motor, a reduction ratio also has a limit.
On the contrary, in case of the low speed rotation which is necessary in the enlarging mode, the stability of the vibration or the like of the optical system causes a problem. With respect to the vibration of the optical system, since the output levels of the line devices of the CCD are almost equal for a monochrome original, the vibration components of every line are set off to a certain degree and the optical system becomes stable. However, since the component of a certain one line device is directly generated for a color original, the vibration components are generated as noises.
Even in the allowable range of the rotational speed of the motor, the motor or the casing to fix the motor has a resonance frequency, so that a vibration occurs at a special frequency or rotational speed and the optical system becomes unstable.
In case of using the method of zooming by using the page memory, in dependence on a magnification for zooming, when the arithmetic operating process such as interpolation or thinning-out is executed, a dropout or error of the output signal occurs. Therefore, a very complicated circuit construction is needed to raise the precision.